Dosage-dispensing devices for pulverous substances are used in particular for applications where small dosage quantities are dispensed with high precision into small target containers. Such target containers are often set on a balance in order to weigh the dosage substance delivered from the dosage-dispensing device, so that the dispensed substance can subsequently be processed further according to given instructions.
The substance to be measured out is located for example in a dosage-dispensing unit which has a source container and a dispensing head. It is desirable that the dosage substance be delivered to the outside through a small delivery orifice of the dosage-dispensing unit, so that the substance can be filled in a targeted stream into a container with a small aperture cross-section.
Many pulverous substances flow almost by themselves in an even stream out of a narrow delivery orifice, comparable to the flow of the sand in an hourglass. In this case, the mass flow rate of the substance can be controlled simply by changing the aperture cross-section of the delivery orifice with a suitable aperture valve device. However, the particles in many pulverous substances have a tendency to stick together. Furthermore, most powders are compactible, so that lumps of compacted powder will form in the dosage substance, whereby the delivery orifice can be partially or completely clogged up. As an example, the particles of cornstarch have a strong tendency to stick together. In addition, cornstarch is very compactible. Consequently, in view of these properties, cornstarch is used frequently as a test material for evaluating the performance of dosage-dispensing devices. The problems caused by the cohesion and compactibility of the dosage material, also referred to as the “stickiness” of a dosage material, have led to numerous proposals for a solution and types of dosage-dispensing devices or, more specifically, their dispensing heads. There are for example dispensing heads with conveyor screws serving to move the sticky dosage material from the source container to the delivery orifice of the dispensing head. The known state of the art further includes stirring mechanisms and scraping devices serving to scrape the dosage material from the inside walls of the source container and the dispensing head and to propel it towards the conveyor screw. The stirring mechanisms can further serve to prevent the formation of so-called bridges of the dosage material in the source container. The known state of the art further includes tapping and vibrating devices which are used instead of, or in combination with, a stirring mechanism.
A possible solution for the aforementioned problems is disclosed in US 2006/011653 A1. A dosage-dispensing device for doses of pulverous substances in the range from a few milligrams to several grams with a precision of better than +/−5% of the target weight has a dosage-dispensing unit which includes in essence a source container and a dispensing head. The inside wall of the dispensing head is shaped with a conically narrowing taper from the juncture with the source container towards the delivery orifice. In the operating state of the apparatus, the delivery orifice is located at the underside of the dosage-dispensing unit and thus below the source container. In the operating state of the apparatus, free-flowing dosage material inside the dosage-dispensing unit will flow towards the delivery orifice under the influence of gravity. The delivery orifice is equipped with an aperture-controlling valve which serves to regulate the mass flow rate of the dosage material delivered to the outside. The dosage-dispensing device includes means for vibrating and/or tapping the container and can further include a stirring mechanism which can in addition be moved up and down in a straight line along its central longitudinal axis or axis of rotation.
In tests that were conducted with cornstarch it has been found that delivering the latter from dosage-dispensing units of a variety of configurations can cause considerable difficulties. The flow capability of this type of a dosage substance can depend directly on how loosely it is agglomerated, a property which is also referred to as bulk density. Some of the experiments could have the outcome that the dosage material was compacted instead of loosened when a stirring mechanism and/or vibrating means was employed. This had the result that in spite of a completely opened aperture orifice, no dosage material could be delivered out of the dosage-dispensing unit.